Fix reading null struct property value in gRPC JSON transcoding

[JamesNK]

Fixes #51521

Reading null struct property values was erroring because a Struct requires null Value. Fix by converting null reference to a null Value by updating the converter to handle null.

[JamesNK]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[JamesNK]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0: https://github.com/dotnet/aspnetcore/actions/runs/7040801995

[alrz]

I suppose because the fix is on Value converter [null] will work as well. I've found another converter that handles null but it's not configured properly: EnumConverter

[ghost]

Hi @alrz. It looks like you just commented on a closed PR. The team will most probably miss it. If you'd like to bring something important up to their attention, consider filing a new issue and add enough details to build context.